you also have issues with Font Awesome icons. This is because of changes in how Font Awesome in integrated with Backlight.
See: Upcoming Changes to Font Awesome
If you want to keep using Font Awesome you have a few options. One is to buy Font Awesome Pro and integrate it with backlight through custom css.
Font Awesome free is also an option. Again, you’ll have to integrate it with Backlight. I don’t remember if Backlight 3 had the option for serving Font Awesome locally.
If you go the Font Awesome Free route, this may be helpful: Font Awesome 5 Free Unicode Fix - TTG Tips and Tricks
By far, the easiest thing to do (probably for both Font Awesome and your original problem) is to upgrade to Backlight 5. It’s kept up to date with php versions so you shouldn’t run into more compatibility issues as php changes.